2-MN is known as a raw material for vitamin K.sub.3 and the like. It is also useful as a raw material for 2,6-naphthalenedicarboxylic acid, a monomer for high performance polyester resins whose development is under progress in recent years. 2-MN is contained in naphthalene oil, obtained from coal tar, and also in heavy oil, available upon cracking of petroleum. 2-MN is concentrated by distillation out of mixtures consisting of itself and co-existing components such as 1-MN, biphenyl and acenaphthene. Of these co-existing components, separation of 2-MN from 1-MN is particularly difficult.
In addition, 2-IPN is a compound useful as a raw material for .beta.-naphthol and the like. As raw materials for the production of 2-IPN, IPN mixtures are well-known. IPN mixtures can be produced industrially by using naphthalene and propylene as raw materials to conduct operations including alkylation, transalkylation and distillation.
Furthermore, 2,6-DIPN is a compound useful as a raw material for 2,6-naphthalenedicarboxylic acid, 2,6-dihydroxynaphthalene and 6-hydroxy-2-naphthoic acid, all known as monomers for high performance polyester resins. As raw materials for the production of 2,6-DIPN, DIPN mixtures are well-known. DIPN mixtures are produced industrially by using naphthalene and propylene as raw materials to conduct operations including alkylation, transalkylation and distillation. DIPN mixtures are used in such applications as microencapsulation solvents for carbonless copying paper and electrical insulation oils.
To obtain 2-MN, 2-IPN or 2,6-DIPN from a corresponding mixture of alkyl-substituted naphthalene isomers as described above, it has heretofore been known to use the following methods by way of example.
Namely, proposed methods for the separation and recovery of high-purity 2-MN include a method of combined distillation and crystallization (Japanese Patent Application Laid-Open No. 95923/1982), a method using an adsorbent (Japanese Patent Application Laid-Open No. 88432/1984) and a method involving distillation subsequent to the addition of an alkanolamine (Japanese Patent Application Laid-Open No. 153233/1987).
However, the above methods were all developed by improving efficiencies of methods whose principles had been known. Their application for the separation of isomers having extremely close physical and chemical properties, such as 1-MN and 2-MN, inevitably leads to the need for large and complex facilities, so that an economical disadvantage is unavoidable. Regarding the separation of 2-MN and 1-MN, there has hence been a demand for the development of a novel method which is more advantageous than the conventional separation methods.
In addition, as methods for separating 2-IPN from an IPN mixture, there have been known a method of melt crystallization disclosed in Japanese Patent Application Laid-Open No. 70347/1975 and a method of cooling and crystallization in a lower alcohol solvent disclosed in Japanese Patent Publication No. 23406/1981.
However, the above conventional methods both require crystallization at a temperature as low as -30.degree. C. Therefore, they require not only a powerful cooling apparatus but also a high utility cost. They are hence not suitable for industrial practice. In addition, 2-IPN tends to form fine crystals. This leads to another problem that extreme difficulties are encountered when attempting to obtain 2-IPN in uniform crystals large enough to permit filtration and washing. It is therefore necessary to pay special attention to the conduction of heat and stirring within a crystallization vessel and the scraping of the crystals deposited on the vessel walls. This leads to a further problem that an exclusively-designed crystallization vessel is needed.
Furthermore, as a method for separating 2,6-DIPN from a DIPN mixture, it has been known to obtain high-purity 2,6-DIPN by cooling the DIPN mixture to crystallize 2,6-DIPN and then collecting the resultant crystals by filtration as disclosed in Japanese Patent Application Laid-Open No. 69055/1975. In addition, Japanese Patent Application Laid-Open No. 88141/1988 discloses the separation of 2,6-DIPN from an isopropyl-naphthalene mixture containing both 2,7-DIPN and 2,6-DIPN by using the formation of an adduct between 2,6-DIPN and thiourea.
However, any attempt at industrially practicing the cooling method of an oily DIPN mixture in the process disclosed in Japanese Patent Application Laid-Open No. 69055/1975 requires cooling of the mixture to a temperature below zero. This cooling therefore requires high initial and utility costs. Moreover, it is necessary to make crystals have a uniform and appropriate size in order to facilitate their filtration and washing. This requires delicate and minute adjustments with respect to the conduction of heat and stirring within a crystallization vessel and the scraping of crystals deposited on the vessel walls. This leads to another problem that an exclusively-designed crystallization vessel is needed. Moreover, to obtain 2,6-DIPN having a purity as high as 99% or higher, it is indispensable to combine plural steps of operations for crystallization into a complex process. Accordingly, an economical disadvantage is inevitable.
On the other hand, the method disclosed in Japanese Patent Application Laid-Open No. 88141/1988 takes a long time for the formation of the adduct. Nevertheless, the purity of 2,6-DIPN to be obtained is lower than 98%. It is therefore not fully satisfactory as an industrial method for the production of 2,6-DIPN which is required to have a high level of purity as a monomer source.